Distortion in the modulation of a voltage-controlled oscillator (VCO) is a well-known problem. Frequently, a non-linear shaping function is employed to shape the modulating signal of a VCO for reducing distortion in the frequency modulation (FM) component of the output signal of the VCO.
One prior art shaping function utilizes a diode-resistor network such as that shown in FIG. 1. The resistors are typically selected for reduced distortion in VCO output which is time consuming and expensive. Distortion specifications for VCO's which employ such a network, range from about -29 dB to about -54 dB.
In another prior art shaping network shown in FIG. 2, the V.sub.GS -I.sub.D characteristic of a field-effect transistor (FET) is used to pre-distort the modulating signal of a VCO to reduce distortion. Since the FET operates in the saturated region of its characteristic, the non-linearity achievable for modulation of the VCO is essentially uncontrollable because it cannot be reduced below a computable minimum. Thus, VCO output distortion is typically no better than about -30 dB.
In the present invention, the non-linear resistance of an FET is employed as one leg of a resistive voltage divider network. The output of this network is also used to pre-distort a VCO modulation signal. However, this network relies on the V.sub.DS -I.sub.D characteristic of the FET which is biased to operate in the linear region of that characteristic. Therefore, nearly any degree of controllable pre-distortion may be achieved.
A pre-distortion shaping network constructed according to the principles of the present invention provides a second order voltage transfer function, the first and second order coefficients of which may be independently selected. The non-linearity of that voltage transfer function is approximately equal to the inverse of the VCO frequency modulation transfer function non-linearity.
The shaping network of the present invention provides at least 30 dB improvement in VCO FM distortion. Typically, -90 dB distortion is obtainable.